Dynamic distortion control

ABSTRACT

A distortion monitor for a non-linear device is provided. The control circuit includes an input coupleable to receive a signal from the non-linear device and a first frequency monitor coupled to the input. The frequency monitor monitors the level of one of even and odd order distortion at a first frequency and creates a first signal indicative of the level of the distortion without the use of a pilot tone.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to commonly assigned, co-pending applicationSer. No. 09/479,298, entitled “PRE-DISTORTER WITH NON-MAGNETICCOMPONENTS FOR A NON-LINEAR DEVICE,” filed on the same date as thepresent application. The '739 Application is incorporated herein byreference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to the field oftelecommunications and, in particular, to dynamic distortion control.

BACKGROUND

Due to the increased demand for higher capacity in voice, data and videotransmission the communications market is expanding. In particularoptical fiber communications technology has been developing in responseto the market requirements. Optical transmitters are one type of fibercommunications technology that is evolving to meet the increased demand.

Optical transmitters that utilize pre-distortion devices for distortioncancellation are well known. Typically, these transmitters are designedaround a Mach-Zehnder optical modulator. The modulator is fed from ahigh power laser. The laser operates in the cool white mode and providesthe “light source” that has its intensity or amplitude modulated in theMach-Zehnder device.

The optical modulation is accomplished by feeding a radio frequency (RF)modulating signal to the appropriate port of the modulator. In this wayRF amplitude modulation is converted to optical amplitude modulation.

A detrimental characteristic of the optical modulator is that itsoptical output/RF input transfer characteristic is very non-linear; itis sinusoidal in nature. Consequently for a large modulation index (theratio of the peak variation actually used to the maximum designvariation (e.g., that variation whereby the instantaneous amplitude ofthe modulated carrier reaches zero) severe odd order distortion isgenerated. In order to overcome this distortion an external means isrequired to compensate for the nonlinear transfer function.

Pre-distorters have been used to in the past to minimize odd orderdistortions generated in the modulator. These odd order distortions arereduced by a circuit that generates its own RF distortions and theninjects them into the modulator out of phase with those that aregenerated by the modulator. The pre-distorters have been limited intheir operating bandwidth and in their absolute distortion cancellationdue to the use of magnetic components used to achieve the phaseinversions that operate to cancel the distortion products.

Typically, an optical transmitter also includes some circuitry tomonitor the distortions introduced in the transmitter. It has beencommon practice to use one or more auxiliary pilot tones along with themany main carrier signals and to monitor distortion products from thepilot tones to asses the operation of the transmitter. One drawback ofthis technique is that it uses additional, unwanted signals. Further,these signals produce distortion products with very low energy, makingthe signals difficult to pick up.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art foran improved mechanism to control distortion in a non-linear device,e.g., an optical transmitter.

SUMMARY

The above mentioned problems with non-linear devices and other problemsare addressed by the present invention and will be understood by readingand studying the following specification. A non-linear device isprovided which uses a distortion monitor to monitor distortion productsgenerated at least in part by transmission of carrier signals to controlthe operation of the non-linear device.

In particular, an illustrative embodiment of the present inventionincludes a distortion monitor for a non-linear device, e.g., an opticaltransmitter. The distortion monitor includes an input coupleable toreceive a signal from the non-linear device and a first frequencymonitor coupled to the input. The frequency monitor monitors the levelof one of even and odd order distortion at a first frequency and createsa first signal indicative of the level of the distortion without the useof a pilot tone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of a transmitter including adistortion monitor according to the teachings of the present invention.

FIG. 2 is a graph that illustrates an embodiment of a process forgenerating a control voltage to control the distortion in a modulatoraccording to the teachings of the present invention.

FIG. 3 is a block diagram of another embodiment of a transmitterincluding a distortion monitor according to the teachings of the presentinvention.

FIG. 4 is a block diagram of an embodiment of a distortion monitoraccording to the teachings of the present invention.

FIG. 5 is a block diagram of an embodiment of a system including atransmitter with a distortion monitor according to the teachings of thepresent invention.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawingswhich form a part of the specification. The drawings show, and thedetailed description describes, by way of illustration specificillustrative embodiments in which the invention may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be used andlogical, mechanical and electrical changes may be made without departingfrom the scope of the present invention. The following detaileddescription is, therefore, not to be taken in a limiting sense.

FIG. 1 is a block diagram of an embodiment of a transmitter, indicatedgenerally at 100, including a distortion monitor 108 according to theteachings of the present invention. Transmitter 100 receives RF inputsignals at input 102 and produces optical outputs at 110 and 112. Inother embodiments a single output or a plurality of outputs areprovided. The RF input signals of transmitter 100 pass through signalpath 104, pre-distorter 106, and modulator 114. In one embodiment,signal path 104 includes at least one amplifier, and an equalizer. Inother embodiments, signal path 104 also includes an attenuator and othercircuits to prepare the RF signal for transmission over an opticalfiber.

Transmitter 100 converts the electrical, RF signals received at input102 to optical signals produced by laser 116 and optical modulator 114.In one embodiment, optical modulator 114 comprises a non-linearmodulator such as a Mach-Zehnder modulator. Laser 116 is coupled tomodulator 114 to create the optical signals at outputs 110 and 112.Unfortunately non-linear modulators introduce distortion in the opticalsignal, e.g., even order and odd order distortion, produced at outputs110 and 112. Thus, transmitter 100 includes circuitry that is used tocompensate for these distortions produced by optical modulator 114.

To provide this compensation, transmitter 100 includes pre-distorter106. In one embodiment, pre-distorter 106 is constructed according tothe teachings of the '739 application. In other embodiments,pre-distorter 106 is constructed using more conventional pre-distortercircuits incorporating magnetic phase inversion circuits. Pre-distorter106 is coupled between signal path 104 and modulator 114. Pre-distorter106 adds distortion to the RF signals from signal path 104 in order tocompensate for the distortion introduced by modulator 114.

Transmitter 100 also includes circuitry that selectively adjusts theoperation of one of pre-distorter 106 and modulator 114 based ondistortions detected in the output of modulator 114. Distortion monitor108 is coupled to receive the output of modulator 114 atfilter/amplifier 118. Filter/amplifier 118 is coupled to first andsecond frequency monitors 120 and 122, respectively. First and secondfrequency monitors 120 and 122 provide signals to microprocessor orcontroller 124. Microprocessor 124 uses a control algorithm, such as thealgorithm described below with respect to FIG. 2, to selectivelygenerate signals for pre-distorter 106 and modulator 114. In oneembodiment, these control signals from microprocessor 124 are used tocontrol a bias voltage for amplifiers in pre-distorter 106 and a DC biasvoltage for modulator 114.

First and second frequency monitors 120 and 122 are tuned to monitorselected frequencies for potential distortion products at these selectedfrequencies. In one embodiment, these distortion products monitored byfirst and second frequency monitors 120 and 122 are generated based onactual signals received at input 102 and are not generated based onpilot tones. In another embodiment, one of the first and secondfrequency monitors 120 and 122 monitors signals generated, at least inpart, based on a pilot tone.

In one embodiment, transmitter 100 operates with cable television (CATV)radio frequency (RF) input carrier frequencies. In a typical system,these frequencies are spaced apart at nominally 6.0 MHZ increments. Theactual 6 MHZ frequency difference between carriers depends on theabsolute frequency accuracy of each individual carrier. This carrieraccuracy is such that the difference of each carrier from the nominal 6MHZ is typically within zero to less than 30 kHz. Modulator 114 convertsthese RF carrier signals to their corresponding optical counterparts.

In one embodiment, first frequency monitor 120 monitors even-orderdistortions and second frequency monitor 122 monitors odd-orderdistortions. One way in which the even-order distortions manifestthemselves follows the f_(n)−f_(n+1) rule whereby an approximate 6 MHZdistortion product is generated for every pair of RF frequencies thatare spaced apart by 6 MHZ. This results in many distortion products inthe frequency spectrum at outputs 110 and 112 of transmitter 100. Firstfrequency monitor 120 detects these even-order distortion productswithin the nominal 6 MHZ±30 kHz frequency range. This provides a measureof the even-order distortion of transmitter 100.

Similarly, one way in which odd-order distortions manifest themselvesfollows the f₁−f₂−f₃ rule whereby a 49.25 MHZ distortion product isgenerated for many combinations of RF frequencies in a multi-channelformat. Again, this results in many distortion products in the frequencyspectrum at outputs 110 and 112 of transmitter 100. Second frequencymonitor 122 detects these odd-order distortion products within thenominal 49.25 MHZ±30 kHz frequency range. This provides a measure of theodd-order distortion of transmitter 100.

In another embodiment, distortion monitor 108 monitors distortionproducts generated by a pilot tone provided to pre-distorter 106. Inthis embodiment, odd-order distortions manifest themselves following thef_(n+1)−f_(n)+f_(m) rule whereby a 49.25 MHZ distortion product is againgenerated for every pair of RF frequencies that are 6 MHZ spaced whenmixed with a single 43.25 MHZ pilot signal (f_(m)). Again, secondfrequency monitor 122 detects these distortion products within thenominal 49.25 MHZ±30 kHz frequency range. This also provides a measureof the odd-order distortion of transmitter 100, with the use of a singlepilot tone.

In operation, transmitter 100 receives electrical RF signals at input102 and produces optical signals at outputs 110 and 112. The signalsfrom input 102 are amplified and prepared for pre-distorter 106 atsignal path 104. Pre-distorter 106 adds a selected distortion to thesignals from signal path 104. In one embodiment, pre-distorter 106 addsodd-order distortions that are inversely related to the expecteddistortions for modulator 114. Laser 116 and modulator 114 combine topass the signals from pre-distorter 106 to outputs 110 and 112.

The amount of distortion in outputs 110 and 112 of transmitter 100 iscontrolled by a feedback loop that monitors the distortion in theoutputs 110 and 112 and provides control signals to pre-distorter 106and modulator 114. This feedback loop includes distortion monitor 108.Distortion monitor 108 receives the output of modulator 114 atfilter/amplifier 118. This signal is further provided to first andsecond frequency monitors 120 and 122, respectively. First frequencymonitor 120, in one embodiment, monitors energy at a first selectedfrequency, e.g., 6 MHZ, that relates to even-order distortion products.Similarly, second frequency monitor 122 monitors, in one embodiment,energy at a second selected frequency, e.g., 49.25 MHZ, that relates toodd-order distortion products. It is noted that in some embodiments apilot tone is used to generate at least some of the odd-order distortionproducts detected by second frequency monitor 122.

Distortion monitor 108 provides two output signals to microprocessor124. First frequency monitor 120 provides a signal that is related tothe amount of even order distortion in the output of modulator 114.Second frequency monitor 122 provides a signal that is related to theamount of odd-order distortion in the output of modulator 114.

Microprocessor 124 uses the signals from distortion monitor 108 toselectively create control signals for pre-distorter 106 and modulator114. For example, microprocessor 124 selectively generates a controlsignal for pre-distorter 106 that controls at least one bias current foran amplifier in pre-distorter 106. Microprocessor 124 also selectivelygenerates a second control signal for modulator 114 that controls a DCbias voltage for modulator 114.

Microprocessor 124 implements a control algorithm depicted graphicallyin FIG. 2 to generate these control signals for pre-distorter 106 andmodulator 114. For purposes of simplicity, the graph of FIG. 2 isdescribed in terms of generating the control signal for modulator 114.However, it is understood, that a similar routine is used to generate acontrol voltage for pre-distorter 106.

At the beginning of the algorithm, microprocessor 124 outputs an initialvalue labeled as V₁ in FIG. 2. This voltage is a preset value thatpresupposes an acceptable setting for modulator 114. Due to temperatureand component aging, the setting will not normally be the final settingestablished by microprocessor 124. At the initial setting, V₁,distortion monitor 108 provides a signal indicating to microprocessor124 that distortion level is D₁. Because this is a single measurement,the algorithm implemented by microprocessor 124 does not know where thispoint lies on curve 202. Thus, microprocessor 124 adjusts the controlvoltage to a value V₂ that is higher or lower than V₁. Microprocessor124 than receives an updated distortion reading, D₂, from distortionmonitor 108. In FIG. 2, distortion level D₂ is depicted as being lowerthan the distortion level D₁. Thus microprocessor 124 determines thatthe control voltage was correctly lowered to reduce the distortion leveland again lowers the control voltage to voltage V₃. It is noted thatwhen lowering the control voltage results in a higher distortion level,microprocessor 124 raises the control voltage to try to move thedistortion level lower. Once microprocessor 124 has determined that thecontrol voltage is moving in the correct direction, microprocessor 124continues to modify the control voltage until the change in the controlvoltage results in increasing distortion such as the situation depictedwith voltage V₆. At this point, microprocessor 124 returns the controlvoltage to the level preceding level that increased distortion, e.g.,V₅. Microprocessor 124 further continuously uses this process to outputcontrol voltages as changes in the distortion level are detected inorder to maintain a control voltage that keeps distortion at a relativeminimum.

FIG. 3 is a block diagram of another embodiment of a transmitter,indicated generally at 300, including a distortion monitor 316 accordingto the teachings of the present invention. The transmitter receives RFinput signals at RF input 301 and the signals are fed to anamplification circuit 303 where the signals are amplified and fed to anequalizer 304. The equalizer 304 receives the amplified signalsequalizes the signals and feeds them to a first coupler 306. The firstcoupler 306 samples the equalized signals and feeds the sampled signalsto a second coupler 303 which splits the sampled signals fortransmission to an attenuator 351 and an internal RF level monitor 304.The attenuator 351 receives the split signals, attenuates them and feedsthe signals to an external RF monitor through RF output 302. Theinternal RF level monitor 304 receives the split signals and feeds thesignals to a microprocessor 312 for monitoring and control.

In addition, the coupler 306 feeds the equalized signals to anattenuator 308. The attenuator 308 receives the equalized signals andattenuates the signals. In one embodiment the attenuator feeds theattenuated signals to a pre-distorter driver 310. In other embodimentsthe attenuator 308 feeds the attenuated signals to a combiner 307 andthe signals are then fed to a pre-distorter driver 310. The combiner 307adds a pilot tone as detectable distortion to the attenuated signals.The pre-distorter driver 310 receives the signals, amplifies the signalsand feeds the amplified signals to a pre-distorter 305.

The pre-distorter 305 generates odd order distortions for input to amodulator 311. In one embodiment the modulator 311 is an opticalmodulator and in another embodiment the modulator is a Mach Zehndermodulator. The odd order distortions generated by the pre-distorter 305are complimentary to distortions generated by the modulator 311. Thecomplimentary signals are substantially equal in magnitude to thedistortions for modulator 311 but are 180 degrees out of phase with thedistortions of modulator 311. The pre-distorter 305 feeds the signalswith the complimentary distortions to the modulator 311. The modulator311 is coupled to a laser bias and temperature control device 314 whichprovides a light source for modulation by the modulator 311. Themodulator 311 receives the RF signals and the light source and generatesmodulated optical outputs 320 and 322. The modulator is nonlinear andproduces odd-order distortions in addition to the optical outputs 320and 322. For a large modulation index the odd-order distortions aresevere. The complimentary odd order distortions created by thepre-distorter 305 substantially reduce the distortions created by themodulator 311.

In this embodiment a distortion monitor 316 monitors the output signal322 of the modulator 311 for the presence of distortion products at oneor more frequencies, e.g., even and odd order distortion products. Thedistortion monitor 316 receives the output of modulator 311 and providesthe monitored distortion levels to the microprocessor 312 to generatecontrol signals. The microprocessor 312 also receives signals from theRF level monitor 304. The microprocessor 312 provides output signalsbased on input signals from RF level monitor 304 to an attenuationcontrol device 305 to control the operation of attenuator 308.

In this embodiment the transmitter 300 is a 1550 nm wavelength externalmodulation transmitter. In other embodiments, transmitter 300 comprisesan optical transmitter that uses direct modulation. In furtherembodiments, other wavelengths are used.

FIG. 4 is a block diagram of an embodiment of a distortion monitor,indicated generally at 408, according to the teachings of the presentinvention. Distortion monitor 408 monitors the output of an opticalmodulator, such as a Mach Zehnder modulator, to determine the presenceof even- and odd-order distortion products. Distortion monitor 408includes first frequency monitor 420 and second frequency monitor 422.First and second frequency monitors 420 and 422 each include a mixerthat is controlled by local oscillator circuits 432. First frequencymonitor 420 is tuned to identify distortion products located in the 6MHZ range. Similarly, second frequency monitor 422 is tuned to identifydistortion products located in the 49.25 MHZ range.

Distortion monitor 408 includes input 430 that is coupled to circuitrythat prepares the output of an optical modulator for input to the firstand second frequency monitors 420 and 422. The signals received at input430 are detected by photo diode 434. Photo diode 434 is coupled to lowpass filter 436 to filter out as much of the carrier frequency energyabove 50 MHZ as possible. This reduces the load on the front end ofdistortion monitor 408 since the carriers are typically 60 to 70 dBgreater than the distortion products. Low pass filter 436 provides itssignal to amplifier 438. Amplifier 438 provides, for example, 20 dB ofdistortion gain and establishes the signal-to-noise ratio of distortionmonitor 408.

The output of amplifier 438 is split into two paths; a first pathpassing through first frequency monitor 420 and a second path passingthrough second frequency monitor 422. In both paths, the output ofamplifier 438 is down-converted to low audio frequency “base-band”signals by mixing the output of amplifier 438 with 6 MHZ and 49.25 MHZsignals, respectively, from local oscillator circuit 432 usingdouble-balanced mixers. The base-band signals are further processed toproduce output signals indicative of the distortion products found inthe output of the optical modulator.

First frequency monitor 420 monitors distortion products located at 6MHZ±30 kHz. First frequency monitor 420 includes low pass filter 440 andamplifier 442 that are coupled in series between amplifier 438 and mixer444. Low pass filter 440 rejects signals above 6 MHZ. Amplifier 442further boosts the level of the distortion products in this frequencyrange. Mixer 444 down-converts the distortion products to base-band.

The output of mixer 444 is coupled to the series combination offilter/amplifier 446, full wave rectifier 448, filter/amplifier 450, andlog amplifier 452. In one embodiment, filter/amplifier 446 has a 30 kHzbandwidth to simultaneously limit the noise power and also allow for asmuch distortion energy as possible to drive full wave rectifier 448. Inone embodiment, filter/amplifier 450 comprises a 1 Hz active filter thatfilters the output of full wave rectifier 448. Log amplifier 452increases the operating dynamic range of distortion monitor 408 bycompressing the normally large signal swing that would result from usinga linear high gain amplifier. The final DC output level of log amplifier452 represents a measure of the even-order distortion products of theoptical modulator. Small DC levels at the output of log amplifier 452represents lower levels of distortion products.

Second frequency monitor 422 monitors distortion products located at49.25 MHZ±30 kHz. Second frequency monitor 422 includes bandpass filter454, amplifier 456, and notch filter 458 that are coupled in seriesbetween amplifier 438 and mixer 460. The pass band of bandpass filter454 includes the 49.25 MHZ frequency. Amplifier 456 further boosts thelevel of the distortion products in this frequency range. In oneembodiment, notch filter 458 is included to suppress carriers above the49.25 MHZ frequency, e.g., the 55.25 MHZ carrier, in order to pass the49.25 MHZ distortion products. Mixer 460 down-converts the distortionproducts to base-band.

The output of mixer 460 is coupled to the series combination offilter/amplifier 462, full wave rectifier 464, filter/amplifier 466, andlog amplifier 468. In one embodiment, filter/amplifier 462 has a 30 kHzbandwidth to simultaneously limit the noise power and also allow for asmuch distortion energy as possible to drive full wave rectifier 464. Inone embodiment, filter/amplifier 466 comprises a 1 Hz active filter thatfilters the output of full wave rectifier 464. Log amplifier 468increases the operating dynamic range of distortion monitor 408 bycompressing the normally large signal swing that would result from usinga linear high gain amplifier. The final DC output level of log amplifier468 represents a measure of the odd-order distortion products of theoptical modulator. Small DC levels at the output of log amplifier 452represents lower levels of distortion products.

Local oscillator circuit 432 provides inputs to mixers 444 and 460 offirst and second frequency monitors 420 and 422, respectively. Localoscillator circuit 432 includes first and second crystal oscillators 470and 472. In one embodiment, the first crystal oscillator 470 comprises a49.25 MHZ oscillator and second crystal oscillator 472 comprises a 43.25MHZ crystal oscillator. First and second crystal oscillators 470 and 472receive a control signal labeled AUTO_ZERO that is used to turn theoscillators on or off.

First crystal oscillator 470 generates the local oscillator signal formixer 460. First crystal oscillator 470 includes an output that iscoupled through amplifier 474, filter 476, and power divider 478 to aninput of mixer 460. This input of mixer 460 receives a 49.25 MHZ localoscillator signal.

First and second crystal oscillators 470 and 472 combine to provide alocal oscillator input for mixer 444. Second crystal oscillator 472includes an output that is coupled to the series combination ofamplifier 480, filter 482, and power divider 484. Power dividers 478 and484 each provide an input to mixer 486. Mixer 486 provides a localoscillator signal, e.g., a 6 MHZ signal, to mixer 444 through filter 488and amplifier 490.

In one embodiment, second crystal oscillator 472 also is used to providea pilot tone to be injected into the signal provided to the modulator.This pilot tone is taken from power divider 484 and selectively providedto the modulator by RF switch 492 under the control of a signal labeledOSC-CTRL.

FIG. 5 is a block diagram of an embodiment of a system, indicatedgenerally at 500, including transmitter 502 with distortion monitor 503according to the teachings of the present invention. In one embodiment,system 500 transports cable television (CATV) channels, e.g., 80 to 112channels, between two distant locations via an optical link. The opticallink is preferred over a coaxial copper cable link because the fiberoptic link provides a wider bandwidth and lower loss.

System 500 includes transmitter 502 that is coupled to receive multicastchannel insertion, e.g., RF signals between 50 and 870 MHZ. Transmitter502 comprises, for example, the transmitter shown in FIG. 1 or 3 above.Transmitter 502 is coupled to distortion monitor 503 which monitorsdistortion products in the output of transmitter 502 and providescontrol signals to transmitter 502 in order to reduce distortionproducts in its output. Distortion monitor 503, in one embodiment, isconstructed as shown and described above with respect to FIGS. 1, 2, 3,and 4.

Transmitter 502 provides output to splitters 504 and 506 to increase thenumber of paths that can be supported by transmitter 502. Each pathsupported by transmitter 502 includes a fiber-optic cable 508. In oneembodiment, fiber-optic cable 508 has a length from 20 to 25 kilometers.Fiber-optic cable 508 is terminated by a receiver 510 that converts theoptical signal back to an electrical signal for transmission overcoaxial cable.

CONCLUSION

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover any adaptations or variations of the presentinvention. For example, the distortion monitor can monitor distortionproducts at any appropriate frequency and is not limited to distortionproducts at 6 MHZ and 49.25 MHZ. Rather, the specific frequency ischosen to be a frequency outside the channel raster alignment at afrequency representative of even or odd order distortions. Further, thecomponents in the first and second frequency monitors can be adjusted asnecessary to allow for monitoring distortion products at selectedfrequencies. Further, any appropriate local oscillator arrangement canbe used to create the local oscillator signals for the first and secondfrequency monitors. Further, in some embodiments, only one frequencymonitor is provided, e.g., to monitor even or odd order distortion at asingle frequency. In other embodiments, the signals from the distortionmonitor are provided as output for use in manual control of thenon-linear device. Further, in other embodiments, the distortion monitoris used with non-linear devices other than an optical transmitter.

What is claimed is:
 1. A distortion monitor for a non-linear device, thedistortion monitor comprising: an input coupleable to receive a signalfrom the non-linear device; and a frequency monitor, coupled to theinput, that monitors the level of one of even and odd order distortionat a first frequency and that creates a signal indicative of the levelof the distortion without the use of a pilot tone.
 2. The distortionmonitor of claim 1, wherein the non-linear device comprises an opticaltransmitter.
 3. The distortion monitor of claim 1, and furtherincluding: a pre-distorter coupled to the non-linear device; and acontroller, coupled to the frequency monitor to receive the first andsecond signals and to selectively create at least one control signal forone of the non-linear device and the pre-distorter.
 4. The distortionmonitor of claim 3, wherein the controller generates first and secondcontrol signals, wherein the first control signal controls a biasvoltage for the pre-distorter and the second control signal controls aDC bias for the non-linear device.
 5. The distortion monitor of claim 1,wherein the frequency monitor includes at least one filter and a mixerthat select the frequency and down convert the frequency to base-band.6. The distortion monitor of claim 1, wherein the frequency monitormonitors distortion products at a frequency outside the channel rasteralignment that are representative of even order distortion.
 7. Thedistortion monitor of claim 1, wherein the frequency monitor monitorsdistortion products at a frequency outside the channel raster alignment,wherein the frequency is representative of odd order distortion.
 8. Acontrol circuit for dynamic distortion control in a non-linear device,the control circuit comprising: an input coupleable to receive a signalfrom the non-linear device; a frequency monitor, coupled to the input,that monitors the level of odd order distortion at a frequency and thatcreates a signal indicative of the level of the distortion without theuse of a pilot tone; and a controller, coupled to the frequency monitorto receive the first and second signals and to selectively create atleast one control signal to control the non-linear device.
 9. Thecontrol circuit of claim 8, wherein the non-linear device comprises anoptical transmitter.
 10. The control circuit of claim 8, and furtherincluding a pre-distorter coupled to the non-linear device andcontrolled by the controller.
 11. The control circuit of claim 10,wherein the controller generates first and second control signals,wherein the first control signal controls a bias voltage for thepre-distorter and the second control signal controls a DC bias for thenon-linear device.
 12. The control circuit of claim 8, wherein thefrequency monitor includes at least one filter and a mixer that selectsthe frequency and down converts the frequency to base-band.
 13. Thecontrol circuit of claim 8, wherein the frequency monitor monitors firstdistortion products at a frequency outside the channel raster alignment,wherein the frequency is representative of odd order distortion.
 14. Acontrol circuit for dynamic distortion control in a non-linear device,the control circuit comprising: an input coupleable to receive a signalfrom the non-linear device; a frequency monitor, coupled to the input,that monitors the level of even order distortion at a frequency and thatcreates a signal indicative of the level of the distortion without theuse of a pilot tone; and a controller, coupled to the frequency monitorto receive the first signal and to selectively create at least onecontrol signal to control the non-linear device.
 15. The control circuitof claim 14, wherein the non-linear device comprises an opticaltransmitter.
 16. The control circuit of claim 14, and further includinga pre-distorter coupled to the non-linear device and controlled by thecontroller.
 17. The control circuit of claim 16, wherein the controllergenerates first and second control signals, wherein the first controlsignal controls a bias voltage for the pre-distorter and the secondcontrol signal controls a DC bias for the non-linear device.
 18. Thecontrol circuit of claim 14, wherein the frequency monitor includes atleast one filter and a mixer that select the frequency and down convertsthe frequency to base-band.
 19. The control circuit of claim 14, whereinthe frequency monitor monitors distortion products at a frequencyoutside the channel raster alignment, wherein the frequency isrepresentative of even order distortion.
 20. A control circuit fordynamic distortion control in a non-linear device, the control circuitcomprising: an input coupleable to receive a signal from the non-lineardevice; a first frequency monitor, coupled to the input, that monitorsthe level of one of even and odd order distortion at a first frequencyand that creates a first signal indicative of the level of thedistortion without the use of a pilot tone; a second frequency monitor,coupled to the input, that monitors the level of the other of even andodd order distortion at a second frequency and that creates a secondsignal indicative of the level of the distortion; and a controller,coupled to the first frequency monitor and second frequency monitors,that receives the first and second signals and that selectively createsat least one control signal to control the non-linear device.
 21. Thecontrol circuit of claim 20, wherein the non-linear device comprises anoptical transmitter.
 22. The control circuit of claim 20, and furtherincluding a pre-distorter coupled to the non-linear device andcontrolled by the controller.
 23. The control circuit of claim 22,wherein the controller generates first and second control signals,wherein the first control signal controls a bias voltage for thepre-distorter and the second control signal controls a DC bias for thenon-linear device.
 24. The control circuit of claim 20, wherein thefirst frequency monitor includes at least one filter and a mixer thatselect the first frequency and down converts the frequency to base-band.25. The control circuit of claim 20, wherein: the first frequencymonitor monitors first distortion products at a frequency outside thechannel raster alignment, wherein the frequency is representative of oneof odd or even order distortion; and the second frequency monitormonitors second distortion products at a frequency outside the channelraster alignment, wherein the frequency is representative of one of oddor even order distortion.
 26. The control circuit of claim 20, whereinthe first and second frequency monitors include double balanced mixers.27. The control circuit of claim 20, and further including a pilot tonegenerator that selectively adds distortion detectable at the secondfrequency.
 28. An apparatus, comprising: an input, coupleable to receivean RF signal; a pre-distorter, coupled to the input, that selectivelyadds distortion to the RF signal; a non-linear device, coupled to thepre-distorter, that receives the RF signal from the pre-distorter toproduce an output for apparatus; wherein the distortion added by thepre-distorter is controlled to reduce distortions in the output of theapparatus generated by the non-linear device; a distortion monitor,coupled to the output of the non-linear device, that monitors at leastone frequency of the output of a transmitter to detect distortion in theoutput without the use of a pilot tone; and a microprocessor, coupled tothe distortion monitor, the pre-distorter, and the non-linear devicethat uses an output of the distortion monitor to selectively generate atleast one control signal for one of the non-linear device and thepre-distorter to reduce the distortion in the output of a transmitter.29. An apparatus, comprising: a non-linear device that receives an inputsignal and produces an output signal; and a distortion monitor, coupledto the output of the non-linear device, that monitors at least onefrequency of the output signal of the non-linear device to detectdistortion without the use of a pilot tone.
 30. The apparatus of claim29, and further including a microprocessor, coupled to the distortionmonitor, that provides at least one control signal to the non-lineardevice.